Approximately 70,000 individuals worldwide suffer from the debilitating disease cystic fibrosis (CF), which typically is fatal owing to sepsis in the abnormally viscous mucus of the lung. Various treatments are employed to address symptoms, but an approved drug that acts on the cause of CF is urgently needed. The cause is the ?F508 mutation of the CF (CFTR), which aberrantly regulates levels of salts inside and outside cells, leading to the disease pathologies. The mutation causes improper folding of the CFTR protein, so that most of it is degraded in the endoplasmic reticulum (ER) in the cell post-synthesis (ERAD) and does not reach the cell membrane. CFTR?F508 that does localize to the membrane is partially functional, and drugs called correctors (e.g., VX-809) are being developed to promote trafficking of mutant CFTR to the cell surface. One of the ERAD ubiquitin E3 ligases that target CFTR is the RING ligase gp78, and knockdown of gp78 leads to dramatically increased levels of CFTR?F508 in cells. Moreover, cells containing catalytically dead gp78 are characterized by reduced ubiquitylation of CFTR?F508. Due to upregulation of CFTR?F508 levels, gp78 inhibition has the potential to increase the amount of CFTR at cell surfaces, especially in combination with correctors such as VX-809. It is thus proposed that gp78 is a novel target for developing small molecule inhibitors for treating CF by increasing levels of functional CFTR, thereby improving the regulation of mucus in the lungs of CF patients. Progenra has developed an assay technology that has identified small molecule inhibitors of various RING E3 ligases; these inhibitors are currently undergoing pre-clinical development. It is proposed here to adapt and validate this assay for screening for inhibitors of human gp78 and conduct a pilot screen for small molecules that can be developed as therapeutic agents for CF. Autoubiquitylation and substrate ubiquitylation assays will be developed, 40,000 compounds from a diversity library will be screened, and suitable inhibitors will be identified based on potency and selectivity as determined by counter-screening of the hits. An appropriate cellular model will be established and cell proof of concept studies initiated on selected inhibitors. In Phase II, more extensive screening will be conducted and selected lead gp78 inhibitors will undergo preclinical development (chemical optimization, DMPK, in vivo efficacy studies) for treatment of CF singly or in combination with correctors such as VX-809. PUBLIC HEALTH RELEVANCE: Although various treatments are employed to address symptoms of the debilitating disease cystic fibrosis (CF), an approved drug that acts on the cause of CF is urgently needed. A protein called CFTR is responsible for maintaining proper salt balance at cell membranes and in CF it is mutated and degraded, leading to imbalance and overly mucous membranes that become septic leading to mortality. Progenra proposes to develop a small molecule inhibitor of the enzyme (gp78) that degrades this mutated protein to restore salt balance and achieve therapeutic benefit.